11,976 research outputs found
Adsorption of CO on a Platinum (111) surface - a study within a four-component relativistic density functional approach
We report on results of a theoretical study of the adsorption process of a
single carbon oxide molecule on a Platinum (111) surface. A four-component
relativistic density functional method was applied to account for a proper
description of the strong relativistic effects. A limited number of atoms in
the framework of a cluster approach is used to describe the surface. Different
adsorption sites are investigated. We found that CO is preferably adsorbed at
the top position.Comment: 23 Pages with 4 figure
Orbital optimization in the perfect pairing hierarchy. Applications to full-valence calculations on linear polyacenes
We describe the implementation of orbital optimization for the models in the
perfect pairing hierarchy [Lehtola et al, J. Chem. Phys. 145, 134110 (2016)].
Orbital optimization, which is generally necessary to obtain reliable results,
is pursued at perfect pairing (PP) and perfect quadruples (PQ) levels of theory
for applications on linear polyacenes, which are believed to exhibit strong
correlation in the {\pi} space. While local minima and {\sigma}-{\pi} symmetry
breaking solutions were found for PP orbitals, no such problems were
encountered for PQ orbitals. The PQ orbitals are used for single-point
calculations at PP, PQ and perfect hextuples (PH) levels of theory, both only
in the {\pi} subspace, as well as in the full {\sigma}{\pi} valence space. It
is numerically demonstrated that the inclusion of single excitations is
necessary also when optimized orbitals are used. PH is found to yield good
agreement with previously published density matrix renormalization group (DMRG)
data in the {\pi} space, capturing over 95% of the correlation energy.
Full-valence calculations made possible by our novel, efficient code reveal
that strong correlations are weaker when larger bases or active spaces are
employed than in previous calculations. The largest full-valence PH
calculations presented correspond to a (192e,192o) problem.Comment: 19 pages, 4 figure
Electronic Excitations in Complex Molecular Environments: Many-Body Green's Functions Theory in VOTCA-XTP
Many-body Green's functions theory within the GW approximation and the
Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP
software, aiming at the calculation of electronically excited states in complex
molecular environments. Based on Gaussian-type atomic orbitals and making use
of resolution of identify techniques, the code is designed specifically for
non-periodic systems. Application to the small molecule reference set
successfully validates the methodology and its implementation for a variety of
excitation types covering an energy range from 2-8 eV in single molecules.
Further, embedding each GW-BSE calculation into an atomistically resolved
surrounding, typically obtained from Molecular Dynamics, accounts for effects
originating from local fields and polarization. Using aqueous DNA as a
prototypical system, different levels of electrostatic coupling between the
regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid
to charge-transfer (CT) excitations in adenine base pairs. It is found that
their energy is extremely sensitive to the specific environment and to
polarization effects. The calculated redshift of the CT excitation energy
compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which
matches expectations from experimental data. Predicted lowest CT energies are
below that of a single nucleobase excitation, indicating the possibility of an
initial (fast) decay of such an UV excited state into a bi-nucleobase CT
exciton. The results show that VOTCA-XTP's GW-BSE/MM is a powerful tool to
study a wide range of types of electronic excitations in complex molecular
environments
Multi-component symmetry-projected approach for molecular ground state correlations
The symmetry-projected Hartree--Fock ansatz for the electronic structure
problem can efficiently account for static correlation in molecules, yet it is
often unable to describe dynamic correlation in a balanced manner. Here, we
consider a multi-component, systematically-improvable approach, that accounts
for all ground state correlations. Our approach is based on linear combinations
of symmetry-projected configurations built out of a set of non-orthogonal,
variationally optimized determinants. The resulting wavefunction preserves the
symmetries of the original Hamiltonian even though it is written as a
superposition of deformed (broken-symmetry) determinants. We show how short
expansions of this kind can provide a very accurate description of the
electronic structure of simple chemical systems such as the nitrogen and the
water molecules, along the entire dissociation profile. In addition, we apply
this multi-component symmetry-projected approach to provide an accurate
interconversion profile among the peroxo and bis(-oxo) forms of
[CuO], comparable to other state-of-the-art quantum chemical
methods
Calculating Hyperfine Couplings in Large Ionic Crystals Containing Hundreds of QM Atoms: Subsystem DFT is the Key
We present an application of the linear scaling Frozen Density Embedding
(FDE) formulation of subsystem DFT to the calculation of isotropic hyperfine
coupling constants (hfccs) of atoms belonging to a guanine radical cation
embedded in a guanine hydrochloride monohydrate crystal. The model systems
considered range from an isolated guanine to a 15,000 atom QM/MM cluster where
the QM region is comprised of 36 protonated guanine cations, 36 chlorine anions
and 42 water molecules. Our calculations show that the embedding effects of the
surrounding crystal cannot be reproduced neither by small model systems nor by
a pure QM/MM procedure. Instead, a large QM region is needed to fully capture
the complicated nature of the embedding effects in this system. The
unprecedented system size for a relativistic all-electron isotropic hfccs
calculation can be approached in this work because the local nature of the
electronic structure of the organic crystals considered is fully captured by
the FDE approach
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